I agree with the person above - the complete subject of this sentence is D. a group of men.
        
                    
             
        
        
        
 [He has estimated that the problem would lead to a loss of $10,000 and would require time to resolve.]
This should be the right answer hope it helps!!
 
        
                    
             
        
        
        
B.  Because if a speaker spoke quickly listeners might have a hard time listening to what they are saying.